In modern aircraft, the primary flight display screens now include a three-dimensional synthetic representation of the outside world. These representations may comprise an indicator of the flight plan followed by the aircraft. Older representations are of the “highway in the sky” type. FIG. 1 shows one of these representations. The trajectory of the aircraft A is symbolically represented by a central path 1. This path 1 is framed by a succession of rectangles 2 representing the limits of the “tunnel” representing the three-dimensional trajectory 1. The track on the ground of the trajectory is represented by the shadow 3.
These representations work well for trajectories perfectly defined within a terrestrial frame of reference. They are referred to as being “georeferenced”. Unfortunately, it is not possible to perfectly georeference the entirety of the trajectory of an aircraft. When the trajectory is not perfectly defined within a terrestrial frame of reference, the portions of the tunnel of the trajectory which are not defined within a terrestrial frame of reference then produce discontinuities, as shown in FIG. 1.
Specifically, in a flight plan, the trajectory may be defined in various ways. As stated above, the simplest way is the planned and georeferenced trajectory, generally computed by the flight management system.
However, in certain phases of the flight, pilots implement flight modes that distance the aeroplane from this planned trajectory.
By way of first example, air traffic control may request that the crew keep to a particular heading or descend to a specific altitude. In this case, the representation of the trajectory is no longer clear. FIG. 2 shows a view from above of a situation in which the aeroplane is guided in heading mode. Aircraft A must be guided towards heading C. The trajectory T effectively being followed is a prediction which includes a portion of rallying towards the target heading, then keeping to the heading. This prediction requires wind to be taken into account, since the heading represents the direction of the nose of the aeroplane, rather than the direction of the path. It may therefore turn out to be imprecise. The hashed portion represents the variation T′ in the trajectory depending on the wind strength.
By way of second example, FIG. 3, which represents a view from above the overflown terrain, shows the difference in trajectory depending on whether a navigation mode is armed. In “armed” mode, when the aeroplane is close to the planned trajectory TP, this trajectory becomes the reference for guidance. The overall prediction for the trajectory T of the aircraft therefore includes a first, predicted portion T1 and a second portion T2 defined within a georeferenced frame of reference and which neighbours the planned trajectory TP. If the navigation mode is not armed, the aircraft continues to follow its initial heading and its trajectory T includes a second portion T′2 differing from the portion T2.
By way of third example, certain portions of the flight may be undertaken in a mode referred to as “performance” mode, in which engine speed is fixed, and the resulting ground slope may vary depending on the wind.
The above concepts explained in relation to lateral trajectory are also valid for vertical trajectory. For example, the aircraft may be guided in “vertical speed” mode, i.e. a mode in which the vertical speed of the aircraft is kept at a precise value. Here again, the prediction is affected by taking the wind into account. As a general rule, a vertical mode is engaged at a setpoint altitude that ends the descent or ascent. The predicted trajectory of the aircraft is then stabilized at this altitude.
Thus, when the trajectory is not perfectly defined within a terrestrial frame of reference, the portions of the tunnel of the trajectory which are not defined within this terrestrial frame of reference produce discontinuities, as shown in FIG. 1.